Vision 350® & Vision 130® Pulse Train Outputs

Motion Control Made Easy

The increasing drive for product quality requires more accurate and consistent movement
from the components of modern day machines. To move accurately and quickly
requires the use of a servo or stepper motor, a device which needs to be told which
direction to move in, at what speed and for precisely how far. The requirement
to be able to alter these parameters easily for different products puts a strain
on traditonal PLC equipment which is not good at handling and processing large amounts
of integer values.

Unitronics engineers have worked hard to reduce the pain involved in motion control
by producing a set of function blocks that allow you to configure movement profiles
using drop down boxes and multiple choice selection. The net result is a neat
little box in the middle of your program that outputs a pulse train to your motor
control card.

There are six elements to the pulse train output (PTO) function block set

Configuration

Set profile

PTO Move

Stop

Set home

Read status

Available only on certain Unitronics Vision® models (those with high speed transistor
outputs!) the Configuration function block is shown below. This block defines
the hardware system and converts pulses to engineering units, up to three channels
may be used and there are three modes available

Clockwise - Counter clockwise

Pulse

Pulse & direction

Having defined the physical system we now set up the move profile which consists
of three stages, acceleration, steady speed and deceleration. The Set Profile
function block allows entry of start speed, stop speed, acceleration and deceleration
times together with a 'jerk factor'. There are six available jerk factors
which specify whether the acceleration and deceleration profiles have straight line
characteristics or use a smoother S Curve approach.

The parameters in a profile may be fixed numerical values or internal registers
if the value needs to change under program or recipe control. Unitronics Vision®
OPLC's have internal data tables and can retrieve and save data onto an SD card
which allows many different profiles to be accessed.

The last part of defining a move is the actual move itself, using the PTO Move function
block the user specifies the direction (a positive value for one direction, a negative
valur for the other direction), the speed and the target position. It is also possible
to specify ABSOLUTE or RELATIVE movement, absolute movement being to a specific
point and relative movement being another movement in the specified direction.

The last three elements of the function block set provide simple but convenient
ways of implementing

Stop

Instructs the motor control card to either slow down and stop according to the current
profile parameters, or to stop immediately following an emergency.

Set home

Send the workpiece to a predefined home position with one simple instruction.

Read status

Monitor where the workpiece is and how fast it is moving.

Motion control programs written in a modern PLC often take up vast areas of code
clogging up the memory and distracting from the original objective of controlling
the machine. Using a Unitronics function block not only makes the whole process
simpler and easier to construct but it removes the clutter of incidental code leaving
the control program nicely structured.